Just in time for Father’s Day, Elon Musk’s tunneling construction company began handing out its first batch of $500 flamethrowers this weekend.

What started as an online gag has turned into a marketing ploy worth millions — with the first 1,000 customers receiving the flamethrower — that promotes the Boring Company, Musk’s endeavor to build underground transportation tunnels.

The billionaire inventor and chief executive of Tesla and SpaceX announced the sale of the flamethrower last year. Only 20,000 of them would be made, he said, and they quickly sold out. “When the zombie apocalypse happens, you’ll be glad you bought a flamethrower. Works against hordes of the undead or your money back!” Musk joked in January, when customers were still able to order them online. At $500 each, the flamethrowers brought in $10 million.

The fire-breathing device comes packaged with a rhyming set of terms and conditions that nods to the classic Dr. Seuss book “Green Eggs and Ham.” “I will not use this in a house. I will not point this at my spouse. I will not use this in an unsafe way. The best use is creme brulee.”

Musk said the device was renamed Not a Flamethrower, “due to recent regulatory/customs rules enacted to inhibit transport of anything called a Flamethrower.”

Some customers are already trying to turn a profit on the device. On eBay, interested buyers can bid on several listings for the Boring Company’s Not a Flamethrower. (Late Monday morning, the cheapest one was priced at $2,250.)  A listing for just the instruction manual was asking for $250. The most eager customers can forgo the bidding process and pay the eBay “buy it now” price, which is as high as $20,000.

For reasons that may seem obvious, especially if you have seen pictures of exuberant customers wielding their flamethrowers, Musk has been criticized for hawking a device that poses a safety risk. The Boring Company is based in Southern California, a region ravaged by wildfires. And some see Musk’s latest ploy as particularly reckless and insensitive.

One customer tweeted a picture of himself posing joyfully in front of the company’s logo, clutching a flamethrower that had just belched a fireball. “I bought something completely awesome and frivolous, so I’m going to also donate $1,000 to the California Wildfire Relief Fund,” he said on Twitter.

Musk replied to him: “We don’t think our product will be the cause of any wildfires, but The Boring Company is donating $10k.”